


clutching

by gooey_boop



Category: Undertale (Video Game)
Genre: Artificial Insemination, Cross Sans - Freeform, Force-Feeding, Leviathan AU, Medical Kink, Medical Procedures, Needles, Non-Consensual Body Modification, Non-Consensual Drug Use, Non-Human Genitalia, Other, Rape/Non-con Elements, Sanster, Undertale AU, clutching, cumflation, induced blackout, non consensual medical procurer's
Language: English
Status: In-Progress
Published: 2021-03-02
Updated: 2021-03-02
Packaged: 2021-03-14 18:14:56
Rating: Explicit
Warnings: Rape/Non-Con
Chapters: 1
Words: 4,561
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29795895
Author URL: https://archiveofourown.org/users/gooey_boop/pseuds/gooey_boop
Summary: as inconvenient as an unwanted heat is, it does give a chance to work with some *delicate* testing
Relationships: W. D. Gaster/Sans
Comments: 4
Kudos: 54





	clutching

**Author's Note:**

> inspired by a thread on askellie's twitter-
> 
> I'm planing one more part, I'll add the new tags when that's out OwO
> 
> also, I'll probably change the title when I come up with a better one

Cross panted softly, his back pressed against the back corner of his tank. Everything was hot; he wasn’t sure if it was the water or something else. It didn’t seem right; Gaster liked to keep the water in his tanks as cold as possible, he said it was too slow movement but Cross was pretty sure he just liked seeing him uncomfortable. 

It must have been early, the main lights in the lab were off; they were set on a timer, turning on around 6:15, and Gaster usually got there 15 minutes after. 

Cross groaned softly, falling onto his side and curling up his tail. Despite the fact he felt like he was being boiled, he couldn’t stop shivering. He growled at the thought that Gaster turned up the temperature to see how hot he would get before he passed out. Cross whimpered softly as the lights flickered on and the tank bubbler garbled to life. He  _ hated  _ that thing so much; Gaster said it was supposed to aerate the water or something but it was just really really annoying. With a soft grumbled Cross let himself float off the floor, flicking his tail once to glide to the other side of the tank, as far away from the insufferable mumbling of the machine.

==========

“Are you awake?” Gaster said, louder than the last time. Cross twitched, his eyes fluttering open but he didn’t move from his spot. Gaster sighed and flicked on the bright light above Crosses tank, making him shift and groan. “Cross. I know you’re awake and I know you can hear me. Come to the surface now or I will drag you up myself”.

Cross shifted and uncurled, letting himself float to the top of the tank. He poked just the top of his head out of the water, pulling his arms around his waist and narrowing his eyes at Gaster. “Your childish actions help no one,” Gaster noted, raising his brow and scanning over the mer. “Shut up” Cross hissed; he could feel his face flushing with warmth as he curled his tail up into his chest.

“Are you feeling odd?” Gaster asked, ignoring the verbal attack as he reached at Cross, pulling him forward by the collar around his neck. Cross hissed as he was dragged to the edge of the tank, flinching as Gaster shown his penlight into one of his eye sockets. He pulled it away, waiting for his pupil to dilate before shining into the other one. “Stop!” Cross croaked, Gaster pulling his arm up to inspect the medicated patch he had placed the day prior. “Hmm” he pressed the back of his hand against Cross’s forehead before dropping him completely. “You feel warmer than normal” he muttered, mostly to himself as he fished a temperature gun out from his desk. Cross hissed softly as the red doot was pointed at his forehead, the device beeping 3 times before Gaster pulled it away. Gaster sighed and walked over to the calendar hanging on the wall, muttering to himself as he looked through the dates “No, not that. It’s too early” he mumbled, rubbing his chin.

Cross sank back under the water, resting the end of his tail against the bottom of the tank. He hissed and flinched as he tried to bend the waist, a jolt of soreness shooting down his back. “Well” Gaster huffed “it must be a fever. You’ve never had a reaction like this to medication prior, and you’re not on anything new”. He shook his head and rolled up his sleeves, going to the other side of the lab to set up for the day.

==========

When Gaster came back, Cross was once again curled up at the bottom corner of the tank, twitching and shivering. His gills were flaring quickly, and the edges of the flaps were white and swollen. “Cross” he sighed, the small creature letting up a stream of bubbles in response “you need to communicate. It’s clear something is wrong. Tell me what.”. Cross yelped as Gaster dipped his arms down into the water, leaning over the edge of the tank to pull him up. Gaster lifted him out of the water, Cross flicking his tail and clawing at Gasters arms to try and get free. 

Out of the water, Cross could clearly feel a thick liquid sliding down the front of his tail. “Hm?” Gasters eyes flicked downwards “ah. That’s why” he grumbled, rubbing the bridge of his nose between his fingers. Cross whined and tried to twist out of Gasters grip as he pulled him out of the tank. “N-no! Stop it stop!” Cross exclaimed twisting and flopping his tail. 

“Stop. if you continue to struggle I will restrain you” Gaster said, Cross continuing to garble. Gaster took Cross to one of the smaller examination tables and dropped him onto the metal. Cross yelped as the cold surface pressed against his body, Gaster rolling him onto his back and holding him down. “Be still. Your heat coming in early is already enough of an inconvenience” he hissed, Cross stopping his trashing. “H-heat?” he croaked, flinching as Gaster pressed two fingers into the skin above his vent. A small amount of milky liquid beaded up and dripped down his stomach. “Yes,” Gaster said, “your mating cycle. So far I have been able to counteract it with simple medication, but I seem to have miscalculated on the timing”. Cross hissed as Gaster slid his thumb shallowly into his vent, drawing out a sharp moan. “Still. As much as I wish I could have prevented this I feel letting it go to waste would be unreasonable”

Cross squeaked as Gaster pulled away “I have been wanting to do some experimentation with new supplies I have obtained. This might be a perfect time”. Cross flinched as Gaster picked him up again, carrying him across the lab and past his main tank. Cross keened softly, clutching to Gaster.

He sat Cross down in a shallow tub, flipping him onto his back. Cross yelped as his center shifted, his body locking up. Gaster pulled open a drawer, shuffling through a pile of cuffs. He took out a few different sizes setting them in the shallow basin. “You will have to stay restrained for an extended period of time” Gaster stated as he snapped one of the smaller cuffs around the base of Cross’s tail, bolting the ring in place. Cross was still in a discombobulated state as another ring was snapped around his torso.

Gaster threaded thick straps through the holes on the cuffs, tying them to the edge of the tub. He pulled Crosses arms out to his sides, tying them down. 

Cross blinked as his coherency began to return “wh-what are you doing?” he whined, twisting in the slight slack. Gaster sighed as he finished with the restraints “due to this being your first unsuppressed heat cycle I expect your reactions will be quite pronounced. I want to keep a constant monitor of you for the next couple of days, especially after I start with posable insemination attempts”.

Crosses eyes widened at that “w-what? What does that mean?” he cried, tugging and splashing at the water. Gaster ignored him as he disappeared into the deeper part of the lab, coming back a few minutes later with a rolling metal tray. He pulled up a chair next to the tub and pulled on a pair of latex gloves. “It seems you have just started this morning, so restraining you before you go completely braindead will be easier for both of us,” he said, taking a syringe from the tray and drawing up a dose of medication.

Cross whined and tugged at his arms and Gaster pulled back his chin, wedging the end of the needle between his vertebrae. He cringed as the medication was injected, stinging slightly. “What was that?” Cross whined softly, falling limp again his binds.

“Something to calm you down” Gaster answered, Cross struggling to keep his breathing under control. This wasn’t the first time Gaster restrained him for testing, but something felt different this time. Something that told him he wasn’t going to be ok with what was about to happen 

==========

Cross gasped back another shallow sob as he trembled. Gaster had kept him in the same position for the last three hours, ignoring his crying and pleads. 

His body felt like it was being cooked for the inside out; his vent was swollen and red, begging for some sort of contact, but the way he was restrained he couldn’t even splash water on it. Gaster had fed multiple tubes up into his gills, a small motor pumping fresh water through them, the cold liquid keeping him painfully aware of his body. His fins twitched and flicked out of his control, tugging at the electrode stickers that were pressed against his body, leading up to a string of monitors that displayed his vitals. 

A thick silicone tube trailed from his nasal cavity up to an IV bag that dripped milky pink liquid down the back of his throat. On his hip, right under where his tail met his ribs Gaster made a small incision and inserted a thin catheter; he had been slowly drawing up samples of a thick translucent liquid, taking them back to the other room for some sort of testing. 

Like clockwork, Gaster would return to Cross every 30 minutes; checking his vitals, drawing blood, and replacing the medication that was being pumped into him. “You seem to be taking this much faster than I thought,” Gaster said as he changed out the bag, hand constructs gently caressing his sides to try and soothe him. Cross whined loudly, rolling his hips slightly and splashing water over the side of the tub. He tried to press himself into the light touches, desperate for contact where it mattered. 

“Calm down,” Gaster said, surprisingly gentle as his constructs faded “I’m trying to get baseline readings and I would prefer to not have to keep you drugged for the next 5 hours”. 

That caught him; “5!?” Cross wailed as Gaster jotted down some notes “hours!?” Gaster sighed and removed the full vial from crosses hip “yes, and possibly longer” he stated flatly, fitting another collective into its place “for my testing to be as effective as possible I want your cycle to be at its earliest peak. Heats in mers can last up to ten days if not dispelled, though I do not intend to need that much time for what I want to do”

Cross gasped on a sob, slamming his head against the bottom of the tank and sending water splashing over the edge “pleassseee!” he begged, thrashing as throwing water all over the floor “I’m sorry! I’m sorry please it hurts! It’s hot please let me go!” he cried, Gaster stepping back to avoid the water. “Whining like a child will do nothing” Gaster hissed, a swarm hands grabbing onto Cross and pinning him down, careful not to pull at any medical equipment. A hand clamped over Crosses face, the smaller breaking out into a high pitch shriek

At that Gaster snapped. 

Cross yelped as a thick needle was jammed into his side. “INSUFFERABLE” he roared “INTOLERABLE” Cross choked as a gag was forced between his teeth “absolutely belligerent!” Crosses vision began to swim, Gasters construct being none too gentle as they wrapped bandages around his tail, pulling them tight and binding them to the edge of the tub. “Absolutely unreasonable! Every time you are required to do a  _ simple  _ task, even just told to sit and be SILENT you have to retaliate! You must know how lucky you are I haven’t removed your ability to retort at  _ all _ ” 

Gaster continued to mutter as he paced back and forth, his hands still working with Cross; one pair floated away to mimic his movements as he rambled loudly. Cross wasn’t listening. Whatever drug Gaster had given him was messing with his perception, the ranting and insults seemingly impossibly far away. 

Cross had snot and tears dripping down his face and neck, trembling and gasping. His tail was almost completely wrapped up mummy-style, the only openings being his gills and vent. Every few wraps the bandages were threaded through the rails on the edges of the tub, effectively immobilizing him from the waist down. 

Gaster halted his pacing, clenching his fists and releasing them with a deep exhale. He stood up straight, regaining his composure. “I have other things I need to work on” he hissed “you are to stay quiet and compliant until I move on to further testing. If you continue to misbehave I will stitch your gills closed and leave you to dry up”. 

Cross garbled against the gag, trying to pry it out with his tongue. The feeling of immobility made him want to scream and thrash, but he couldn’t. He couldn’t even see the ceiling above him as tears streamed down his face.

==========

Somehow Cross had managed to dose off. When he woke, Gaster was leaning over him, smearing a thick gel in the space under his nose. He groaned softly, weakly trying to pull away; Gaster grabbed his chin to hold him still. 

The substance was cold and sticky and had a strong, heavy smell. “You were gone for a long while” Gaster mused, Cross gurgling in response; “You’re lucid? That’s odd” Gaster muttered, finishing with the gel “I expected you to be out for at least another hour”. As Gaster pulled away Cross shook his head slightly, then sneezed. Gaster made a small noise of amusement as he wheeled over a tray and sat in a chair next to the tub, pressing his foot on the lift to raise it slightly. 

Cross scrunched up his face, trying and failing to wipe away the jelly. “W-what is this?” He squeaked, his voice still not fully back. “A pheromone laced topical. I’m hoping it will help through the next phase”. Cross groaned and blinked slowly, trying to get his vision steady he watched Gasters' disembodied hands float around the room, doing simple tasks. Looking over at the wall he could see the clock. 6:50 AM. The next day. He really  _ had _ been out for a long time 

Gaster watched with interest as his subject's focus was drawn away. Strange; usually his attention was engaged on the task at hand, whether or not he was participating willingly. Perhaps a side effect of the Tranquilizers along with the incoherence that normally came with heats. Gaster sighed and pulled on a pair of gloves, squeezing a dollop of lubricant into his hands and rubbing down the length of a slim speculum.

The smaller yelped, jolting at the touch of cold lube at his vent; looking down at himself he could see the tips of his hemiphoids poking out of their pouch. “Hush” Gaster snapped, rubbing lubricant into his entrance “don’t make me gag you again”. Cross hadn’t even noticed it was out. Shifting his position slightly, Gaster inerted the tip of the device into the space below his lower flap, sliding it in slowly.

Cross’s mouth hung open in a silent cry; the stretching was an odd sensation, the metal and lube cold pressing against the sensitive, inner flesh. Gaster very rarely messed with his lower flap, the feeling so strange and foreign it almost hurt. The walls of his passage were swollen and tight, spasming around the object as it was pressed further inside him. 

“Calm down” Gaster muttered, slowly cranking it open. Cross gasped and twisted his shoulders forward, unable to lessen the sensation as he was pried open. Gaster snapped the locking pin into place, taking a syringe of lubricant from the tray, and inserted it as deep as he could, Cross moaning loudly as he squirted it inside him. The cold, slick liquid dribbled further down inside him, welling up at the barrier of his cervix. 

holding his penlight with one hand, Gaster took a long, thin tube with a small split on the ends; he inserted one end into Crosses passage, feeding it further down. When he hit the barrier, it took him a bit of work to slid the end through the narrow passage, Cross croaking and whining at the sharp stinging.

“Calm, down” Gaster reiterated, fitting a syringe into the spit off of the free end of the tube, injecting some water. Cross shivered as it filled the split inside him, locking in place. Gaster tugged it gently, making sure it would hold. “Good boy” he muttered, unlatching the speculum and pulling it out, letting Crosses swollen walls pull closed. “Good boy”.

Cross keened softly, the tips of his fins twitching. Whatever that gel stuff was, was making his eyes water and nose burn; he felt lethargic and heavy and he could do nothing else but lazily watch gaster work. 

Gaster stood from his seat as a pair of hands lifted a large jug of milky white liquid over to the tub, setting it down on the floor and bringing over a large syringe. “W-what are y-you doing” he whimpered, his eyelights flicking between the jug and Gaster. The taller skeleton was at the sink, scrubbing his hands. “As in what am I doing now? Or what did I just do?” he asked calmly, Cross whining softly in response. Gaster sighed and patted his hands down with a towel “I just inserted a straight access line into your egg sack.” he said calming, another pair of hands bringing him over a glass of water “it’s more of a backup in case you don’t take to an artificial mating replication. I can also use it to administer direct nutrition if your body does clutch. Better to do it now” 

Cross watched as the hand constructs undid the lid of the jug, dipping the syringe inside and slowly drawing up the fluid. His soul began to beat faster as he watched them wipe the syringe clean with a paper towel, then smeared medical lubricant over it. “Hyperventilating will not help you,” Gaster said, sending another hand to change out Crosses IV bag to a thinner, clear drip.

Cross whined and whimpered, watching in horror as the large syringe was pressed into his vent, sliding in effortlessly with help for both the lubricant and his own shameful arousal. The soft round insertion felt delectable as it slid inside him, Crosses hips spasming to try and force it deeper. The hands stopped momentarily, then pushed it in just an increment further, pulling back the plunger slightly before depressing it. 

Crosses eyes widened and he yanked at his arms. Warm, thick liquid splurted inside him, seeping into the crevices of his vent. He let out a long wail, tears streaming down his face as he twisted his wrists in the restraints, ringing the bone red. He gasped and sputtered as the syringe retracted, the last dribbles leaking onto his stomach. 

“Hnnnggghh” he whined softly, the construct fitting the syringe off to the sink to rinse it. small beads of the substance dribbled out of him, his hemiphoids pressed insistently against his pouch.

Gaster checked his watch “I’m going to start dosings every half an hour. Give your body some time to absorb it before giving more” it sounded more like he was talking to himself. Either way Cross wasn’t listening; he was focused on the feeling of the liquid squelching inside him, every time he breathed the weight shifted. He desperately wanted to curl into himself; force the liquid deeper and fill his swollen needy hole with anything he could find. But he couldn’t. He couldn’t even shift his tail to get a better sensation. 

Cross sputtered on a sob, Gaster rolling his eyes to go work on something else.

==========

Cross moaned as Gaster slipped another tube into his gills, hooking it up to the water pump. He had been trying to tap out or fall asleep for the last ten or twenty minutes, but apparently that would be too much because Gaster just turned up the pump and added colder water. 

“Hush” he hissed; Cross had been moaning, and whining, and making noise for no reason for the last 30 minutes. He really wanted to gag him, but if he was going to keep up testing he would need to insert a feeding tube at some point. With the amount of energy this project was going to drain from Cross, he would definitely be needing pure nutriment, whether or not his body decided to clutch. And with the way he was currently behaving Gaster was sure pinning him between his knees and forcing a liquid meal down his throat would remind him  _ exactly _ who had the power here.

After warming it, Gasters constructs drew another syringe from the jug, applying a generous amount of lubricant to the circumference before pressing it into Cross. He hitched and whined as it was wiggled inside him, still not deep enough to give him any release. The second load made his stomach bulge, Gaster furrowing his brow and burrowing it deeper before finishing it off. 

“W-wha-t” Cross moaned softly, his hole quivering as it was left empty again. “I said before. I’m attempting artificial insemination. Trying to see if you are capable of carrying young.” he explained, a hand constructs wiping off the syringe. “This” he tapped the jug “is ref mer semen. An associate of mine was able to obtain it. As inconvenient as this cycle is it gives me a chance to use it. It’s not as fresh as it could be, so it most likely will not produce fertile eggs, but if your body can be a viable host I may consider further experimentation”

Cross’s eyelights darted around his sockets as he let the word sink in. “e-e-eggs” he squeaked, “n-no no nonono you can’t”. A new wave of tears spilled down his face as he frantically shook his head “no! Nononono!! Y-y-you c-an’t do-do- tha-t!!” he wailed, smashing the back of his head into the tub, sending water splashing all over the floor. 

Gaster watched him with lidded eyes, letting him wear himself out as he thrashed. When he was done with his outburst, Gaster stepped forward, leaning over Cross. “Oh, but I can,” he said calmly, Cross panting and gasping below him “I can do whatever I want to do. You have no power. I have complete control over every aspect of your life, from the moment I got you, to the day you dust.” he leaned in, Crosses eyelights dilating to pinpricks “Every time you retaliate I want you to know; You. Are. replaceable. If I decide you are better off as an egg factory I will not hesitate to constrain you to an examination tank and turn you into an object. A piece of equipment.”

Crosses breathing became labored and heavy, his eyes rolling into the back of his head as he started to pass out, spit bubbling up from the corners of his mouth. A strong but gentle hand grabbed his face by the cheeks, tilting his head back and forth to try and keep him conscious. Another hand brought a small syringe of glucose to Cross, wedging it between his teeth. “Swallow” Gaster ordered, rubbing his thumb under his chin to encourage his body. 

The sweet syrup stirred Cross back to life, the smallers eyelights dilated to the size of quarters. “That’s it. Eyes open” Gaster said, Cross whimpering and crying. “P-please” he begged softly, “I can’t… I can’t please stop it”. Gaster sighed as another syringe of syrup was squeezed into Crosses mouth. “Begging like a dog will not help you” he hissed, resting his palm against Cross’s tummy, pressing down gently. Cross choked; a small amount of fluid dribbling out of him. “You’re fine. I will be back in half an hour. For now I suggest you think about your situation. it will do you good”

==========

A gentle hand tilted Crosses chin upward, a firm finger rubbing on his throat, encouraging him to swallow as a long tube was fed down his throat. 

His tummy was swollen and tender, his hips and tail sore from being held in place. The edge of his vent was swollen and red; it curled back slightly to reveal his pulsing inner structures. “Good” Gaster cooed softly, taping the free end of the tube to the side of his face. Cross whimpered softly, leaning into the touches as Gaster pet his stomach and sides. His breath hitched as Gaster pressed his fingers into his abdomen, feeling around the area. “Your body is accepting” he muttered, pressing down a litter firmer. 

Cross whimpered at the pressure; Gaster felt clinically around his sides and down his tail. He slipped his thumb into Cross’s vent, lifting the swollen edges to wiggled a little deeper. “Nghhhu” Cross gasped softly, thin liquid gushing out of his entrance. Gaster examined the substance; he flicked his wrist and a hand brought over a small vial and cotton swab. It took a sample of the fluid then fitted it away. 

  
  


“I’ll administer one more dose, just for assurance,” he said, three more hands bringing over the warmed jug and drawing up another syringe. Cross whimpered and watched as the cold lube was rubbed over it. He flinched, moaning softly as it pushed inside him, his walls gripping tightly around it. “After this, I’ll leave you to process; when you begin to clutch I will move you to an examination tank”. Cross sighed as another load of warm thick fluid splirted inside him; the feeling was familiar and comforting, filling his already swollen abdomen to a euphoric pressure. He moaned softly, his eyelids flickering closed; his walls clung tightly to the object as it was pulled out, the last little bits dribbling onto his stomach

==========

Cross stirred from his lethargic state at the feeling of the bandages being unwrapped. He wiggled slightly, thankful for the new freedom. “Be still of i will tie you back up” Gaster growled, winding up the wraps as his hands handed them to him. The large bolts holding the rings were undone and set in the water, followed by the straps threaded to the tub then the electrode stickers. Cross cried softly as he was lifted out of the water, Gaster holding him gently against his chest. 

His hands wheeled the IV stand along with him as he carried Cross across the lab. He set him down gently into the open face of an examination tank. “I’m going to put you to sleep for a while,” Gaster said, manipulating Cross’s limp body to attach equipment. The tubes on his face were attached to the tank; Cross squeaked as a hand tugged on the tube inside his vent, screwing the end into place. 

Gaster closed the front of the tank and pressed a button on the side. Cross flinched slightly, not expecting the warm water that bubbled out of the floor. The tank quickly filled, lifted his limp body up. Cross floated forward slightly pressing his hands against the glass sides to steady himself. His vision began to swim, his head lolling to the side. 

A thin stream of bubbles slipped past his teeth, sweet darkness creeping up from the corners of his eyes to grant him the gift of unconsciousness 

**Author's Note:**

> I'm not totally happy with this, but I'm getting better!


End file.
